1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a method of crystallizing polysilicon, a method of fabricating a thin film transistor using the same, and a method of fabricating a liquid crystal display thereof to form a polysilicon layer having uniformly oriented crystalline grains with high quality.
2. Discussion of the Related Art
Generally, a liquid crystal display (hereinafter abbreviated LCD) for a video apparatus has a variety of uses for a thin film transistor such as a switching device. A semiconductor layer in a thin film transistor (hereinafter abbreviated TFT) is formed of an amorphous silicon layer. There are advantages in small-scaled TFT LCD fabrication, but disadvantages in fabrication of a large-sized TFT LCD due to low electron mobility.
Hence, many efforts have been made to provide a polysilicon TFT using a polysilicon layer having excellent electron mobility as a semiconductor layer. Such a polysilicon TFT can be easily applied to the fabrication of a large-sized TFT LCD and is very competitive in integration and cost since the polysilicon TFT can be integrated on a TFT array substrate together with a driver IC.
The polysilicon layer is formed by various methods such as polysilicon direct deposition, crystallization of amorphous silicon into polysilicon, and the like. Generally, the latter method is used so that an amorphous silicon layer formed on a substrate is changed onto a polysilicon layer by carrying out crystallization thereon.
The former methods include plasma enhanced chemical vapor deposition (PECVD) using SiF4/SiH4/H2 mixed gas at deposition temperature below 400° C. and the like. Yet, PECVD has difficulty in controlling grain growth, whereby the growing direction becomes irregular so as to degrade surface characteristics of a polysilicon film.
The latter methods include solid phase crystallization (SPC) of heating amorphous silicon in furnace for crystallization; excimer laser annealing (ELA) of crystallizing a film by irradiating an excimer laser beam of a high-output pulse laser instantly to apply heat to the film; metal induced crystallization (MIC) of inducing crystallization on amorphous silicon on which metal is deposited selectively by applying an electric field thereto using the metal as a seed; field enhanced metal induced crystallization (FEMIC) developed from MIC, and the like.
ELA, in which strong energy of a short wavelength (λ=0.3 μm) as a pulse form is applied to a silicon layer about 300˜800 Å thick so as to melt the layer. Fast crystallization and excellent crystalline properties are realized to improve electron mobility.
Specifically, the short wavelength of the excimer laser uses the energy concentration of laser beams, thereby enabling precise annealing locally in a short time without causing any thermal damage on a lower silicon layer.
Meanwhile, a grain size of a polysilicon layer formed by ELA can be controlled precisely by varying the thickness of an amorphous silicon layer, the density of ultraviolet (UV) ray irradiation generated from laser, and the temperature of a lower substrate.
Explained in the following drawings are a method of crystallizing polysilicon, a method of fabricating a thin film transistor using the same, and a method of fabricating a liquid crystal display thereof according to a related art.
FIGS. 1A to 1C illustrate cross-sectional views of a process of crystallizing polysilicon according a related art.
Referring to FIG. 1A, silicon oxide (SiO2) is deposited on a glass substrate 10 so as to form a buffer oxide layer 11. An amorphous silicon layer 12 is formed on the buffer oxide layer 11 by depositing amorphous silicon at 300˜400° C. using one of PECVD, low pressure chemical vapor deposition (LPCVD), sputtering, and the like.
The buffer oxide layer 11 prevents particles in the substrate 10 from diffusing into the amorphous silicon layer 12 as well as preventing heat influx into the substrate 10 in a later crystallization process.
Referring to FIG. 1B, an excimer laser beam is irradiated on the amorphous silicon layer 12 to apply instant energy thereto so as to melt the amorphous silicon layer 12. In this case, a growth seed layer 13 which fails to melt exists in a lower portion of the amorphous silicon layer 12.
Thereafter, the melted amorphous silicon layer is solidified to grow crystals so as to be transformed into a polysilicon layer. Crystal growth is brought about centering around the growth seed layer 13. As shown in FIG. 1C, the growth seed layer 13 diffuses by the energy of the laser beam to move in a uniform direction so as to carry out crystallization.
Specifically, the preferred orientation of grain growth depends on the alignment of the growth seed layer. Generally, the crystalline growth orientation prevails mainly in the <111> orientation direction inclining to a substrate and is followed by orientation orders such as <220>, <311>, and the like so as to determine the growth directions of the grain.
Instead, in micro-polysilicon, grains having the <220> orientation direction vertical to a substrate prevail, grains having the <111> orientation direction occupy about 40%, and grains having the <311> orientation direction occupy about 10%.
Various growth orientations of grains exist in the polysilicon layer after laser annealing, whereby growing paths of the respective growing grains are interrupted. Thus, the grains fail to grow with ease. Moreover, grain boundary density increases so as to reduce electron mobility.
A polysilicon TFT according to a related art is fabricated as follows.
First, silicon oxide and amorphous silicon are deposited on a substrate in order so as to form buffer and amorphous silicon layers, respectively. Annealing using an excimer laser is carried out on the amorphous silicon layer so as to crystallize the amorphous silicon layer into a polysilicon layer.
The crystallized polysilicon layer is then patterned to form a semiconductor layer. Impurities are implanted in the semiconductor layer selectively so as to form source/drain regions. In this case, impurity implantation is carried out using a gate electrode insulated from the semiconductor layer as a mask, whereby a region, which is masked by the gate electrode so that the impurities fail to be implanted into the polysilicon layer, becomes a channel region.
Thereafter, source and drain electrodes are formed of metal material so as to be connected to the source and drain regions, respectively. In this case, the source and drain electrodes are insulated from the gate electrode by an insulating layer.
Thus, a polysilicon TFT having polysilicon as the semiconductor layer is completed.
Meanwhile, a liquid crystal display (LCD) having a polysilicon TFT includes a first substrate, a second substrate, and a liquid crystal layer provided between the first and second substrates. The first substrate includes gate and data lines arranged to cross with each other so as to define a pixel area, a polysilicon TFT, and a pixel electrode. The second substrate includes a color filter layer and a common electrode.
Unfortunately, the polysilicon crystallizing method, the polysilicon TFT fabrication method using the same, and the LCD fabrication method thereof according to the related art have the following disadvantages or problems.
Various growth orientations of the respective grains exist in the polysilicon layer crystallized through excimer laser annealing, whereby interference between the growing grains interrupts the grain growth. Also, the grain boundary density increases so as to reduce the mobility of electrons or holes.
Moreover, the liquid crystal display using a polysilicon layer as a channel layer fails to provide good reliability as a large-sized display device having high resolution and definition.